The Phones jack uses

The SEK checks the headphone's impedance when a headphone is plugged in at switching on, or each time a headphone is newly plugged in.

To connect earphones to the SEK:
  1. Turn down the volume.
    The volume can be altered between -100 dB to +27.5 dB in steps of 0.5 dB.
  2. Insert the cable’s 3.5 mm jack plug into the phones socket on the SEK.

To connect an adapter cable:
  1. Insert the 3.5 mm jack plug of the cable into the Phones jack on the SEK.
  2. Connect the other plug to the desired device.
  3. In the WebUI, set the Phones Output to Balanced, see Selecting the phones output.
    The Balanced setting can only be used with mono Link Modes.
The Phones jack is used.